1. Field of the Invention
The present invention relates generally to plating baths and methods for monitoring the trace constituents contained therein. More particularly, the method of the present invention relates to a voltammetric analysis technique that accurately and selectively indicates the level of a particular trace constituent in plating baths containing multiple trace constituents. The method can be used to maintain desired trace constituent concentrations in order to ensure optimal plating bath performance.
2. Description of Related Art
A typical plating bath solution is comprised of a combination of several distinct electrochemical constituents. The specific constituents vary depending upon the type of plating bath, but in general can be broadly divided into what are commonly known as major constituents and trace, or minor, constituents. The major constituents are those electrochemical constituents which make up about 2 to 25 percent of the total bath weight. Trace constituents, on the other hand, are present in smaller quantities, usually less than 0.5 percent of the total weight. Organic addition agents, degradation products and chemical contaminants are typical trace constituents.
Most plating baths contain several distinct trace constituents. Appropriate concentration levels of these trace constituents must be maintained in order to ensure a high quality deposit. Trace constituent concentrations influence certain important characteristics of the deposit, including tensile strength, ductility, solderability, uniformity, brightness and resistance to thermal shock.
In terms of organic addition agents, plating baths frequently contain throwing power enhancers, brighteners, grain refiners, ductility promoters and wetting agents. For example, a Lea Ronal acid cadmium plating bath may contain trace concentrations of the organic addition agents Starter K, Brightener KR, and Stabilizer. Each of these organic addition agents affects various qualities of the resultant plating deposit by adsorbing onto the plated surface.
Current techniques for monitoring the trace constituents of plating baths include real time voltammetric monitoring techniques such as those in U.S. Pat. No. 4,631,116, assigned to the present assignee. The method disclosed therein uses a voltammetric signal to produce ac current spectra which vary as a result of changes in the various trace constituent concentrations. Voltammetric methods such as these have been found to produce accurate results in real time for most trace constituents.
Although well-suited for their intended purpose, the above voltammetric techniques may not yield accurate results for all types of plating baths and the trace constituents contained therein. To ensure accurate measurements and a high quality plating deposit, it is important that the analysis technique adequately distinguish among the various trace constituents. The measurements should be unambiguous and highly selective. For certain plating bath trace constituents, the voltammetric techniques of U.S. Pat. No. 4,631,116 are insufficiently selective to permit optimal plating bath analysis. For example, in the Lea Ronal acid cadmium bath discussed above, first or second harmonic ac voltammetry does not adequately distinguish the various organic addition agent concentrations. Other voltammetric techniques, such as those disclosed in U.S. Pat. No. 4,132,605 and U.S. Pat. No. 4,812,210, are similarly limited in their ability to accurately monitor certain multiple trace constituents present in a single bath.
As is apparent from the above, there presently is a need for a method of selectively monitoring trace constituent concentrations within a plating bath containing certain multiple trace constituents. The method should provide accurate real-time results in situations where the accuracy of known multiple trace constituent analysis techniques is limited. Furthermore, the method should complement and be easily integrated with known voltammetric techniques and equipment, resulting in an efficient and flexible overall plating bath analysis system.